Acoustic transducer

ABSTRACT

A hydrophone is provided by a piezoelectric shear element fastened between two masses in various configurations. Sound waves impinge on and accelerate one mass. The acceleration of the one mass and the inertia of the other mass set up shearing forces in the piezoelectric shear element. The piezoelectric shear element produces a voltage proportional to the shearing forces.

United States Patent [72] Inventor Everett L. Fabian Fort Wayne, Ind.[21] Appl. No. 646,551 122] Filed June 16, 1967 [45] Patented June 8,1971 [73] Assignee The Magnavox Company Fort Wayne, Ind.

[54] ACOUSTIC TRANSDUCER 5 Claims, 13 Drawing Figs.

[52] [1.8. CI. 310/83, 310/8.1, 310/84, 310/85, 310/86, 310/91, 310/94,310/95, 340/10 [51] Int. Cl lllllv 7/00, H04r 17/00 [50] Field of Search340/ 10; 310/80, 8.4, 8.3, 8.7, 8.5, 8.6, 9.1, 9.5

[56] References Cited UNITED STATESPATENTS 2,488,586 11/1949 Diemer310/84 3,104,335 9/1963 Shoor 310/8.4 3,307,054 2/1967 Shoor.... 310/84Primary Examiner-Milton O. Hirshfield Assistant Examiner-Mark O. Budd Attorney- Richard T. Seeger ABSTRACT: A hydrophone is provided by apiezoelectric shear element fastened between two masses in variousconfigurations. Sound waves impinge on and accelerate one mass. Theacceleration of the one mass and the inertia of the other mass set upshearing forces in the piezoelectric shear element. The piezoelectricshear element produces a voltage proportional to the shearing forces.

PATENTEU JUN 8 I97! SHEET 1 [IF 4 F cos9 MASS M2 MASS m,

SHEAR PIEZOELECTRIC LAYER |o POLING/ DIRECTION FIG. I

Fcosgb POLING DIRECTION\ FIG. 2

lfiVE/VTOR.

EVERETT L. FABIAN FIG. 3b

GZIQN-S & FIG. 6a

67 sl znza-w 5 so SZo EN-S FIG. 6b

INVENTOR.

EVERETT L. FABIAN PATENTED JUN 8197i 3584243 SHEET k [1F 4 FIG. 7b

FIG. 8b

F] 8a lNl/E/VTOR.

EVERETT L. FABIAN ATTORNEYS ACOUSTIC TRANSDUCER BACKGROUND OF THEINVENTION My invention relates to an improved electroacoustictransducer, and particularly to such a transducer for use in liquids.

Electroacoustic transducers are used in liquids for converting appliedelectrical energy or signals into acoustical energy or sound fortransmission through the liquid, and for converting acoustical energy orsound in the liquid into electrical energy or signals for use in anelectrical circuit. When operated in the latter mode, such transducersare commonly referred to as hydrophones. I-Iydrophones are extensivelyused in water for detecting acoustical energy which may be relativelyweak and which may have frequencies covering a relatively wide band.Hydrophones are also used to detect the direction of the source ofacoustical energy with respect to some reference direction.

Because of the many uses and applications of hydrophones, there isalways a need and desire for better hydrophones, especially those whichhave directional characteristics. Directional information may beobtained by utilizing two or more normally nondirectional hydrophones ortransducers in spaced arrays. Other hydrophones, such as the velocityribbon type, have directional characteristics because of the nature oftheir construction. The spaced array uses pressure sensitive hydrophoneswhich are spaced so that the pressure gradient of the acoustic signaland its direction relative to the source may be determined. Generally,four such pressure sensitive hydrophones are positioned in a circulararray 90 apart. The two pressure sensitive hydrophones along one axisprovide a voltage indicative of the pressure gradient along the oneaxis, and the two pressure sensitive hydrophones along the other axisprovide a voltage indicative of the pressure gradient along the otheraxis. This spacing technique can be used with many types of hydrophones;however, at relatively low acoustic frequencies, the pressure gradientsare relatively small so that the hydrophones must be spaced relativelyfar apart in order to provide a usable sensitivity. The velocity ribbontype of hydrophone utilizes a metallic strip which is positioned in amagnetic field. An impinging acoustic wave causes movement of the stripwith the result that it produces a voltage in the magnetic field. Twosuch ribbons may be oriented at 90 with respect to each other so as toprovide a directional hydrophone. However, this type of hydrophone isrelatively expensive and is not as rugged and reliable as may berequired in certain applications.

Accordingly, an object of my invention is to provide an improved soundtransducer particularly for use in water.

Another object of my invention is to provide an improved acoustictransducer or hydrophone for producing electrical signals in response tosound energy that covers a relatively wide band of frequencies, theelectrical signals having characteristics indicative of thecharacteristics of the sound energy.

Another object of my invention is to provide an improved acoustictransducer that is relatively rugged and strong, and that is capable offunctioning under various marine and environmental conditions.

Another object of my invention is to provide an improved hydrophonewhich has directional characteristics, and which is easily packaged anddeployed.

Another object of my invention is to provide a transducer device whichis not only easily fabricated, but which also has directionalcharacteristics that are not dependent upon the operational frequency orupon the matching of the sensitivity of separate transducers as inspaced directional arrays.

Some of the prior art electroacoustic transducers or hydrophones aresensitive to rotational velocities. Such velocities may cause erroneousoutput signals to be produced, or may distort or ruin the desired outputsignal so that a useless or erroneous signal is provided.

Accordingly, another object of my invention is to provide an improvedhydrophone which is relatively insensitive to rotational velocitiesabout its center of mass.

SUMMARY OF THE INVENTION Briefly, these and other objects are achievedin accordance with my invention by a hydrophone having at least onepiezoelectric shear type element fastened to two masses. One of themasses is arranged so that it is exposed to sound energy which is to bedetected. This sound energy strikes or impinges on the one mass andaccelerates it so that a shearing force is set up in the piezoelectricshear element because of the inertia of the other mass. This shearingforce causes the piezoelectric shear element to create a voltageindicative of the shearing force. This voltage may be derived byelectrical connections to the usual piezoelectric surface electrodes orto the two masses, where the masses act as such electrodes or areelectrically connected to such electrodes. In a preferred embodiment,the one mass is a hollow cylinder that is closed at its ends by a topand a bottom. The other mass is a solid cylinder positioned coaxiallywithin the hollow cylinder. A first piezoelectric shear element isfastened between one end of the solid cylinder and the top, and a secondpiezoelectric shear element is fastened between the other end of thesolid cylinder and the bottom. The poling direction or axis of greatestsensitivity of the first piezoelectric shear element is preferablypositioned at with respect to the poling direction or axis of greatestsensitivity of the second piezoelectric shear element so that thehydrophone can respond to signals from all directions. Two or morepiezoelectric shear elements may be fastened between one end of thesolid cylinder and the top, and two or more piezoelectric shear elementsmay be fastened between the other end of the solid cylinder and thebottom. The piezoelectric shear elements may be connected so that thehydrophone does not produce signals in response to rotational motionabout its center of mass, but does produce signals in response toimpinging sound waves.

BRIEF DESCRIPTION OF THE DRAWING The subject matter which I regard as myinvention is particularly pointed out and distinctly claimed in theclaims. The structure and operation of my invention, together withfurther objects and advantages, may be better understood from thefollowing description given in connection with the accompanying drawing,in which:

FIG. 1 shows a side elevational view of a simplified embodiment of myinvention;

FIG. 2 shows a top plan view of the simplified embodiment of FIG. I;

FIG. 3a shows a longitudinal cross-sectional view of a preferredembodiment of a hydrophone constructed in accordance with my invention;

FIG. 3b shows a simplified equivalent electrical circuit of thehydrophone of FIG. 3a;

FIG. 4a shows a longitudinal cross-sectional view of another preferredembodiment of a hydrophone constructed in accordance with my invention;

FIG. 4b shows a simplified equivalent electrical circuit of thehydrophone of FIG. 4a;

FIG. 5 shows a typical output response curve with respect to frequencyof the hydrophone of FIGS. 4a and 4b;

FIG. 6a shows an exploded perspective view of a hydrophone constructedin accordance with my invention that is relatively insensitive torotational motion;

FIG. 6b shows a simplified equivalent electrical circuit of thehydrophone of FIG. 6a;

FIG. 7a shows an exaggerated cross-sectional view of the hydrophone ofFIG. 60 when subjected to rotational motion about its center of mass;

FIG. 7b shows a simplified equivalent electrical circuit of a portion ofthe hydrophone under the conditions shown in FIG. 70;

FIG. 8a shows an exaggerated cross-sectional view of the hydrophone ofFIG. 6a when subjected to an impinging sound wave; and

FIG. 8b shows a simplified equivalent electrical circuit of a portion ofthe hydrophone of FIG. 8a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show sideelevation and top plan views of a simplified representation of ahydrophone constructed in accordance with my invention. Although otherconfigurations of the piezoelectric element may be used, my hydrophoneas shown in FIG. 1 comprises a piezoelectric shear type element in theconfiguration of a plate or layer which is connected or fastened to amass M, and a mass M,. The piezoelectric element 10 may be of anysuitable piezoelectric material having the desired shear properties.Examples of such materials are: lead zirconate, lead titanate, sodiumniobate, potassium niobate, or crystalline quartz. As known in the art,these materials, when properly polarized, produce voltages at theirouter electrodes in response to shearing force set up in the material.The materials have a poling direction or a direction of greatestsensitivity as indicated by the arrow in FIG. 1 and in FIG. 2. Thispoling direction is the direction along which a given shearing forcecauses the piezoelectric element 10 to produce the greatest voltage atits electrodes. One such piezoelectric element I0 is commerciallyavailable from the Clevite Corporation of Bedford, Ohio, under thedesignation PTZ-S. With reference to FIG. 1, if a force F is applied tothe mass M, at an angle 0 (Theta) with respect to the poling direction,the mass M, and the mass M, are accelerated in a direction parallel tothe poling direction by a force F cos 0. The acceleration of the mass M,and the inertia of the mass M; introduces a shearing force M: (F cos 0)/(M,+M,) along the poling direction. With respect to FIG. 2, if theforce F is at an angle (Phi) with respect to the poling direction, themass M, and the mass M,, are accelerated in a direction parallel to thepoling direction by a force F cos I With respect to FIGS. 1 and 2, theacceleration of the mass M, and the inertia of the mass M introduces anet shearing force M,j(M,+M (F cos 0 cos b) along the poling direction.Thus, the shear piezoelectric element 10 produces a voltage in responseto a force F in almost all directions relative to the poling direction.The magnitude of this voltage is increased as the ratio M,j(M,+M,) isincreased. Since the effective force parallel to the poling direction ofthe shear piezoelectric layer 10 varies as a function of the cosine ofthe angle of the applied force with respect to the poling direction, theoutput of the piezoelectric element 10 becomes very small as either theangle 0 or the angle 1 approach 90.Conversely, the output of thepiezoelectric element 10 becomes its greatest when both the angle 0 andthe angle b approach zero degrees.

The output voltage V produced by a piezoelectric element 10 may bedetermined by the following equation:

volts/centimeters dynes/eentimeters squared t is the thickness incentimeters of the shear element, M is the mass of the mass that is thelast to be accelerated, F is the component of force acting on mass M 0and I are the angles at which the force F is applied relative to thepoling direction, A is the area of the shear element in squarecentimeters, N is the number of shear elements, and M, is the mass ofthe mass receiving the applied force F (i.e., the first mass to beaccelerated). Calculations for a typical piezoelectric element that is1.3 centimeters long, 103 centimeters wide, and 0.845 centimeter thickindicate that such an element has a sensitivity of -I2O db. (referenceto 1 volt per microbar) for an applied force of 0.132 dyne. Such forcesare typically produced by relatively weak underwater acoustic signals orwaves. Hence, the output voltage ofa piezoelectric shear element ispractical for available acoustic forces, and the sensitivity of theelement may be easily calculated.

FIG. 3a nhows a longitudinal cross section of a hydrophone constructedin accordance with my invention and which uses the principles outlinedin connection with FIGS. 1 and 2. In

FIG. 3a, l provide an inner, solid metallic cylinder 12 which serves asthe mass M, or FIG. 1. This cylinder 12 is positioned concentricallyalong a longitudinal axis 14. At the upper end of the cylinder 12 Iprovide a piezoelectric shear element 16 which may have a left-rightpoling direction in the plane of FIG. 3a. For convenience, this isreferred to as the north-south (N-S) direction. At the lower end of thecylinder 12 I provide a piezoelectric shear element 18 which may have apoling direction perpendicular to the plane of FIG. 3a. For convenienee,this is referred to as the east-west (E-W) direction. These elements 16,18 are fastened to the inner inertial mass 12 by any suitableelectrically conductive securing means such as a conducting epoxy. Acircular metallic upper plate 20 is fastened to the outer face of theelement 16, and is positioned concentrically about the axis 14. Asimilar circular metallic lower plate 22 is fastened to the outer faceof the element 18, and is positioned concentrically with respect to theaxis 14. The upper and lower plates 20, 22 are electrically connected tothe shear elements 16, 18 respectively by any suitable electricallyconductive securing means such as a conducting epoxy. The hydrophone isclosed by a hollow, metallic cylinder 24 which is positioned between andfastened to the upper and lower plates 20, 22 so that it is concentricwith respect to the axis 14. The cylinder 24 is electrically insulatedfrom the plates 20, 22 as indicated by the insulation layers 26, 27,which may be a nonconducting epoxy. The hollow cylinder 24 and theplates 20, 22 act or serve as the mass M of FIG. 1. The inner cylinderor mass 12 is electrically connected to the cylinder 24 by a connection30 which preferably is a soft or pliable material that does not transmitmechanical vibrations between the inner cylinder I2 and the outercylinder 24. A first terminal 32 is connected to the upper plate 20, acommon or second terminal 34 is connected to the cylinder 24, and athird terminal 36 is connected to the lower plate 22. The piezoelectricelements I6, 18 have their respective poling directions lying inparallel planes but positioned at right angles relative to each other.Thus, the hydrophone shown in FIG. 3a is sensitive to signals or soundwaves for 360 around the axis 14, and is also sensitive to sound waveswhich reach the hydrophone at an angle less than with respect to theaxis 14.

FIG. 3b shows a simplified equivalent electrical circuit of thehydrophone in FIG. 3a. The same reference numerals have been used forthe circuit elements in FIG. 3b which correspond to the parts in FIG.3a. In FIG. 3b, it will be seen that voltages or signals from the upperpiezoelectric element 16 may be derived from the terminals 32, 34, andthat voltages or signals from the lower piezoelectric element 18 may bederived from the terminals 34, 36. In FIG. 3b, the capacitors shown bythe dotted lines between the terminals 32, 34 and between the terminals34, 36 represent the capacitance resulting from the upper plate 20 withrespect to the cylinder 24 and the cylinder 12 and their associateddielectrics; and likewise, the lower plate 22 with respect to thecylinder 24 and the cylinder 12 and their associated dielectrics. Thehydrophone shown in FIG. 3a was constructed and tested. The inner massor cylinder 12 was constructed of type metal having a diameter of 1.75inches, and a height of 3.36 inches. The upper and lower plates 20, 22were constructed of aluminum, having a diameter of 3.5 inches and athickness of 0.0625 inch. The cylindrical shell or hollow cylinder 24was also constructed of aluminum having an outside diameter of 3.5inches, a wall thickness of 0.0625 inch, and a height of 3.5 inches. Thecylinder 24 was joined to the plates 20, 22 by a nonconducting epoxyhaving a thickness of approximately 0.03125 inch at the layers 26, 27.The shear piezoelectric layers 16, 18 were Clevite PZT-S having asurface dimension of 0.5 by 0.5 inch, and a thickness of approximately0.1 inch. These layers 16, 18 were fastened to the inner mass orcylinder 12 and to the upper and lower plates 20, 22 respectively by aconducting type of epoxy. The outer surfaces of the assembled hydrophonewere covered with a nonconducting epoxy for insulation. The completedhydrophone was tested up to approxinamely 6,000 cycles per second. Atapproximately 2,300 cycles per second, the hydrophone exhibited a selfresonance.

FIG. 4a shows another hydrophone in longitudinal cross section. Thishydrophone comprises an inner inertial mass or cylinder having two solidmetallic cylinders 41, 42 joined by an insulating or nonconducting layer43 and concentrically positioned on a longitudinal axis 40. Apiezoelectric shear type element 45 is fastened to and electricallyconnected to the upper surface of the cylinder or mass 41, and apiezoelectric shear-type element 46 is fastened to and electricallyconnected to the lower surface of the cylinder or mass 42. As explainedin connection with FIG. 3a, the upper layer 45 has a N-S polingdirection in the plane of the paper of FIG. 4a, and the lower layer 46has an E-W poling direction perpendicular to the plane of the paper.Upper and lower circular metallic plates 48, 50 are concentricallypositioned on the axis 40, and are respectively fastened to andelectrically connected to the piezoelectric elements 45, 46. Thehydrophone is closed or surrounded by a concentric, hollow, cylindricalmetallic cylinder 51 which is electrically connected to the plates 48,50. Electrical signals are derived from a terminal 54 connected to thesolid cylinder 41, a common terminal 55 connected to the hollow cylinder51 or one of the plates 48, 50, and a terminal 56 electrically connectedto the other solid cylinder 42. The outer surfaces of this hydrophoneneed not be insulated from the surrounding liquid transmission medium,but may be so insulated by a suitable material if desired. Acousticsignals which reach the outer cylinder or mass 51 cause shearing forcesto be set up in the piezoelectric elements 45, 46, so that electricalsignals are produced at the terminals 54, 55, 56. FIG. 4b shows asimplified equivalent electrical circuit of the hydrophone of FIG. 4a.The same reference numerals have been used for the circuit elements ofFIG. 4b which correspond to the parts in FIG. 4a. In FIG. 4b, thecapacitor indicated by dotted lines and connected between the two innermasses or cylinders 41, 42 represents that capacitance resulting fromthe area between the two masses 41, 42 and the associated dielectric ofthe insulating layer 43. The coupling or interaction between the N-S andE-W circuits which results from this capacitance may be reduced if aconducting layer is positioned midway between the outer boundaries ofthe insulating layer 43 and connected to the common terminal 55.

The hydrophone of FIG. 4a was constructed and tested. The innercylinders or masses 41, 42 were constructed of type metal having adiameter of 1.75 inches. The cylinders 41, 42 were joined by theinsulation layer 43 having a diameter of 1.75 inches and a thickness ofapproximately 0.0625 inch. The combined length of the joined cylinders41, 42 was 3.0625 inches. The piezoelectric elements 45, 46 were made ofClevite PZT-S having a surface area of 0.5 by 0.5 inch and a thicknessof 0.25 inch, and were fastened by solder. The upper and lower plates48, 50 were constructed of brass having a diameter of 4 inches and athickness of 0.0625 inch. The outer cylinder or mass 51 was alsoconstructed of brass and had an inside diameter of 4.0 inches, a wallthickness of 0.0625 inch, and a height of 3.8125 inches to allow forsolder thickness. The cylinder 51 and plates 48, 50 were solderedtogether. This hydrophone was tested with the outer cylinder and endplates uninsulated from the surrounding liquid transmission medium. Itsresponse with frequency is shown in FIG. 5. It will be noted that thehydrophone of FIG. 4a exhibits a resonance at approximately 2,300 cyclesper second. However, the response of this hydrophone is useful and ofgood quality down to frequencies as low as cycles per second. Generally,the frequency response is a linearly varying one. The directionalresponse of the hydrophone of FIG. 4a, as well as the hydrophone of FIG.3a, generally followed the cosine function mentioned earlier so thattheir directional patterns have the familiar FIG. 8 response. Bothhydrophones exhibited a resonance around 2,300 cycles per mcond. Thisresonance may, if desired, be damped or otherwise controlled by the useof materials for the inner and/or outer masses which inherently exhibitthe desired resonant characteristics and/or by the addition of suitabledamping material to the masses. In addition, the resonant frequency maybe shifted, if desired, by changing those design parameters which affectthe resonant frequency of the masses.

The hydrophones of FIGS. 30 and 4a were constructed with the idea thattheir respective axes 14, 40 would be oriented in a vertical direction.Thus, sound traveling through water in a generally horizontal directionwould strike or impinge on the hydrophones at a right angleto these axes14, 40. If these hydrophones rotate about a horizontal axis passingthrough their center of mass, the acceleration resulting from thisrotation causes the piezoelectric elements to produce a spurious outputsignal. FIG. 6a shows, in an exploded perspective view, a hydrophone inaccordance with my invention which reduces the signals resulting fromthis rotation about a horizontal axis passing through the center ofmass. The hydrophone comprises an inner mass 60 to which piezoelectricshear type elements 61, 61a are fastened and poled in an east-west (E-W)direction. Outer or additional piezoelectric shear type elements 62, 62apoled in a north-south (N-S) direction are respectively fastened to theouter faces of the east-west elements 61, 61a. The poling directions liein parallel planes. End plates 65, 65a are fastened to the outer facesof the northsouth elements 62,62a respectively, and the outer mass iscompleted by an outer cylindrical shell 66. A simplified equivalentelectrical circuit of this hydrophone is shown in FIG. 6b, where circuitelements in FIG. 6b are given the same reference numerals as theircorresponding parts in FIG. 6a. A common terminal 68 is connected to thejunction of the elements 61, 62 and to the junction of the elements 610and 62a. An east-west terminal 67 is connected to the inner mass orcylinder 60. A north-south terminal 69 is connected to the end plates65, 65a and/or the outer mass or cylinder 66. If the hydrophone of FIG.6a is rotated about its center of mass 70 in a clockwise direction asshown by the arrows 71, 72 in FIG. 7a, the east-west piezoelectricelements 61, 61a will produce the voltage polarities shown in FIG. 7a attheir respective electrodes. FIG. 70 also shows in exaggerated form thedirection in which the piezoelectric elements 61, 61a, 62, 62a are movedby the external clockwise rotational force about the center of mass 70.This movement causes the voltages of the elements 61, 61a to have apositive polarity at their upper electrodes,

-and a negative polarity at their lower electrodes. When these voltagesare combined at the terminals 67, 68 as shown in the partial circuitdiagram of FIG. 7b, it will be seen that no voltage exists between theterminals 67, 68. In a similar manner, no voltage would be produced by acounterclockwise rotation about the center of mass 70. Thus, rotationalmotion or acceleration about the center of mass 70 produces no netoutput signal in the combined piezoelectric elements. A similarcondition holds for the north-south elements 62, 62a, but thesepolarities have not been shown in order to keep FIGS. 7a and 7brelatively simple. However, if an acoustic signal or wave strikes thehydrophone as indicated by the arrow in FIG. 8a, the piezoelectricelements take the position indicated in exaggerated form in FIG. 8a. Inthis case, the inner electrodes of the east-west shear piezoelectricelements 61, 61a are negative, and the outer electrodes are positive.When these voltages are combined at the terminals 67, 68, a voltage isproduced as shown in the partial circuit diagram of FIG. 8b. A

voltage is also produced by the north-south layers 62, 620.

when a north-south signal is received. Thus, the use of twopiezoelectric shear type elements for each of the two directions, namelyeast-west and north-south, compensates for any rotational accelerationabout a horizontal axis passing through the center of mass.

It will thus be seen that my invention provides an improved hydrophonewhich can sense sound waves in a liquid such as sea water, and which canbe constructed in a relatively rugged and compact structure. Myhydrophone can be compensated for rotation about horizontal axes by theuse of two shear piezoelectric elements. Rotation about the controlvertical axis can be compensated with shear piezoelectric elements onthat axis or symmetrically located about that axis. My hydrophone isrelatively insensitive to vibrations that compress the piezoelectricshear-type elements, or that set up resonant and symmetrical vibrationsthat are in opposition relative to the longitudinal axis of thehydrophone. My hydrophone also provides an improved response over a wideband of frequencies. While I have shown specific embodiments of myhydrophone, persons skilled in the art will appreciate thatmodifications may be made. For example, the inner and outer masses maybe constructed of other materials such as plastic, and may have othershapes such as spherical. The inner and/or outer masses may be coveredwith material for damping and/or insulation. Other electrical circuitsand electrode configurations for combining the output voltages of thepiezoelectric elements may also be used. The piezoelectric shear-typeelements may have various configurations such as a circular shape ratherthan a square or rectangular shape as shown and described. In additionmore than one piezoelectric element may be used at each end of the innermass for a given poling direction, if additional strength or holdingstructure is desired between the inner mass and the outer mass, or ifdifferent voltage output levels or impedances are desired. Additionalpiezoelectric elements may be used at each end or at one end of theinner mass to provide two or more poling directions. Where theadditional elements are used, they may be stacked or may be in aside-by-side configuration. The outer mass of plates and cylinders neednot be supported by the piezoelectric elements alone, but can besupported to some extent by a mechanical construction between the innermass and the outer mass, if such construction does not reduce theshearing forces below the necessary level. And finally, other fasteningmeans, such as welding or mechanical fasteners, may be used. Therefore,while by invention has been described with reference to particularembodiments, it is to be understood that modifications may be madewithout departing from the spirit of the invention or from the scope ofthe claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An improved electroacoustical transducer for detecting sound energyin a liquid transmission medium comprising: a first cylindrical masshaving a peripheral surface in acoustic contact with said transmissionmedium and arranged to be ac celerated in directions substantially atright angles to the longitudinal axis of said cylindrical mass byimpingement of said sound energy upon said surface, the planes of saidsurface being parallel to said longitudinal axis, a first piezoelectricshear type element having a first shearing portion secured to said firstmass for accelerating said element in response to acceleration of saidfirst mass, said element having a poling direction substantially atright angles to the longitudinal axis of said first cylindrical mass, asecond mass shielded from said sound energy and fastened to a secondshearing portion of said piezoelectric element and providing inertia sothat a shearing force is set up in said piezoelectric element inresponse to acceleration of said first mass, and electrodes coupled tosaid piezoelectric element for deriving an electrical signal therefrom;said second mass being surrounded by said first mass and so positionedthat it is isolated from direct acceleration by said sound energy;further comprising a second piezoelectric shear type element fastened tosaid first and second masses, said first and second piezoelectricelements having poling directions disposed in parallel planes andoriented at substantially 90 with respect to each other.

2. An improved electroacoustical transducer for detecting sound energyin a liquid transmission medium comprising: a first mass having aperipheral surface in acoustic contact with said transmission medium andarranged to be accelerated in directions substantially at right anglesto the longitudinal axis of said mass by impingement of said soundenergy upon said surface, the planes of said surface being parallel tosaid longitudinal axis, a first piezoelectric shear type element havinga first shearing portion secured to said first mass for acceleratingsaid element in response to acceleration of said first mass, saidelement having a poling direction substantially at right angles to thelongitudinal axis of said first mass, at second mass shielded from saidsound energy and fastened to a second shearing portion of saidpiezoelectric element and providing inertia so that a shearing force isset up in said piezoelectric element in response to acceleration of saidfirst mass, electrodes coupled to said piezoelectric element forderiving an electrical signal therefrom, and further comprising a secondpiezoelectric shear type element fastened to said first and secondmasses, said first and second piezoelectric elements having polingdirections in parallel planes and oriented at different angles withrespect to each other.

3. An improved hydrophone comprising: a first mass of material, a firstpiezoelectric shear-type element having one active surface fastened tosaid first mass, a second mass of material fastened to another activesurface of said piezoelectric element, said first mass surrounding atleast a portion of said second mass and said piezoelectric element sothat impinging sound energy accelerates said first mass only to set upstresses in said piezoelectric element and so that said piezoelectricelement produces a voltage which varies as a function of said stresses,and a second piezoelectric shear-type element having one active surfacefastened to said first mass and having another active surface fastenedto said second mass so that impinging sound energy accelerates saidfirst mass to set up stresses in said second piezoelectric element, andso that said second piezoelectric element produces a voltage whichvaries as a function of said stresses, said first and secondpiezoelectric elements have their poling directions lying in parallelplanes but extending at right angles with respect to each other.

4. An improved hydrophone comprising: a cylindrically shaped inner massconcentrically positioned along a longitudinal axis, a firstpiezoelectric shear-type element having one active face operativelyfastened to one end of said inner mass and symmetrically positionedrelative to said longitudinal axis, a second piezoelectric shear-typeelement having one active face operatively fastened to the other end ofsaid inner mass and symmetrically positioned relative to saidlongitudinal axis, said first and second piezoelectric elements havingpoling directions disposed in planes that are substantially parallel toeach other, a hollow outer mass positioned around said inner mass andsaid first and second piezoelectric elements and spaced from said innermass, said outer mass being symmetrically positioned relative to saidlongitudinal axis and being fastened to respective second active facesof said first and second piezoelectric elements so that acceleration ofsaid outer mass in a direction lateral to said axis creates shearingstresses in said first and second piezoelectric elements, meansconnected to said first and second piezoelectric elements derivingelectrical signals therefrom, third and fourth piezoelectric shear-typeelements respectively operatively secured between said first and secondpiezoelectric elements and said outer mass, said third and fourthpiezoelectric elements having poling directions disposed in planesparallel to the planes of the poling direction of said first and secondpiezoelectric elements extending in a first direction and those of saidthird and fourth piezoelectric elements extending in a second direction.

5. An improved hydrophone comprising: a cylindrically shaped inner massconcentrically positioned along a longitudinal axis, a firstpiezoelectric shear-type element having one active face operativelyfastened to one end of said inner mass and symmetrically positionedrelative to said longitudinal axis, a second piezoelectric shear-typeelement having one active face operatively fastened to the other end ofsaid inner mass and symmetrically positioned relative to saidlongitudinal axis, a hollow outer mass positioned around said inner massand said first and second piezoelectric elements and spaced from saidinner mass, said outer mass being symmetrically positioned relative tosaid longitudinal axis and being fastened to respective second activefaces of said first and second piezoelectric elements so thatacceleration of said outer mass in a direction lateral to said axiscreates shearing stresses in said first and second piezoelectricelements. means connected

1. An improved electroacoustical transducer for detecting sound energyin a liquid transmission medium comprising: a first cylindrical masshaving a peripheral surface in acoustic contact with said transmissionmedium and arranged to be accelerated in directions substantially atright angles to the longitudinal axis of said cylindrical mass byimpingement of said sound energy upon said surface, the planes of saidsurface being parallel to said longitudinal axis, a first piezoelectricshear type element having a first shearing portion secured to said firstmass for accelerating said element in response to acceleration of saidfirst mass, said element having a poling direction substantially atright angles to the longitudinal axis of said first cylindrical mass, asecond mass shielded from said sound energy and fastened to a secondshearing portion of said piezoelectric element and providing inertia sothat a shearing force is set up in said piezoelectric element inresponse to acceleration of said first mass, and electrodes coupled tosaid piezoelectric element for deriving an electrical signal therefrom;said second mass being surrounded by said first mass and so positionedthat it is isolated from direct acceleration by said sound energy;further comprising a second piezoelectric shear type element fastened tosaid first and second masses, said first and second piezoelectricelements having poling directions disposed in parallel planes andoriented at substantially 90* with respect to each other.
 2. An improvedelectroacoustical transducer for detecting sound energy in a liquidtransmission medium comprising: a first mass having a peripheral surfacein acoustic contact with said transmission medium and arranged to beaccelerated in directions substantially at right angles to thelongitudinal axis of said mass by impingement of said sound energy uponsaid surface, the planes of said surface being parallel to saidlongituDinal axis, a first piezoelectric shear type element having afirst shearing portion secured to said first mass for accelerating saidelement in response to acceleration of said first mass, said elementhaving a poling direction substantially at right angles to thelongitudinal axis of said first mass, a second mass shielded from saidsound energy and fastened to a second shearing portion of saidpiezoelectric element and providing inertia so that a shearing force isset up in said piezoelectric element in response to acceleration of saidfirst mass, electrodes coupled to said piezoelectric element forderiving an electrical signal therefrom, and further comprising a secondpiezoelectric shear type element fastened to said first and secondmasses, said first and second piezoelectric elements having polingdirections in parallel planes and oriented at different angles withrespect to each other.
 3. An improved hydrophone comprising: a firstmass of material, a first piezoelectric shear-type element having oneactive surface fastened to said first mass, a second mass of materialfastened to another active surface of said piezoelectric element, saidfirst mass surrounding at least a portion of said second mass and saidpiezoelectric element so that impinging sound energy accelerates saidfirst mass only to set up stresses in said piezoelectric element and sothat said piezoelectric element produces a voltage which varies as afunction of said stresses, and a second piezoelectric shear-type elementhaving one active surface fastened to said first mass and having anotheractive surface fastened to said second mass so that impinging soundenergy accelerates said first mass to set up stresses in said secondpiezoelectric element, and so that said second piezoelectric elementproduces a voltage which varies as a function of said stresses, saidfirst and second piezoelectric elements have their poling directionslying in parallel planes but extending at right angles with respect toeach other.
 4. An improved hydrophone comprising: a cylindrically shapedinner mass concentrically positioned along a longitudinal axis, a firstpiezoelectric shear-type element having one active face operativelyfastened to one end of said inner mass and symmetrically positionedrelative to said longitudinal axis, a second piezoelectric shear-typeelement having one active face operatively fastened to the other end ofsaid inner mass and symmetrically positioned relative to saidlongitudinal axis, said first and second piezoelectric elements havingpoling directions disposed in planes that are substantially parallel toeach other, a hollow outer mass positioned around said inner mass andsaid first and second piezoelectric elements and spaced from said innermass, said outer mass being symmetrically positioned relative to saidlongitudinal axis and being fastened to respective second active facesof said first and second piezoelectric elements so that acceleration ofsaid outer mass in a direction lateral to said axis creates shearingstresses in said first and second piezoelectric elements, meansconnected to said first and second piezoelectric elements derivingelectrical signals therefrom, third and fourth piezoelectric shear-typeelements respectively operatively secured between said first and secondpiezoelectric elements and said outer mass, said third and fourthpiezoelectric elements having poling directions disposed in planesparallel to the planes of the poling direction of said first and secondpiezoelectric elements extending in a first direction and those of saidthird and fourth piezoelectric elements extending in a second direction.5. An improved hydrophone comprising: a cylindrically shaped inner massconcentrically positioned along a longitudinal axis, a firstpiezoelectric shear-type element having one active face operativelyfastened to one end of said inner mass and symmetrically positionedrelative to said longitudinal axis, a second piezoelectric shear-typeelement having oNe active face operatively fastened to the other end ofsaid inner mass and symmetrically positioned relative to saidlongitudinal axis, a hollow outer mass positioned around said inner massand said first and second piezoelectric elements and spaced from saidinner mass, said outer mass being symmetrically positioned relative tosaid longitudinal axis and being fastened to respective second activefaces of said first and second piezoelectric elements so thatacceleration of said outer mass in a direction lateral to said axiscreates shearing stresses in said first and second piezoelectricelements, means connected to said first and second piezoelectricelements deriving electrical signals therefrom, wherein said first andsecond piezoelectric elements having poling directions that lie inparallel planes but that are positioned at an angle with respect to eachother.